Treating IRS Rogue Pixels with Dark Subtraction

Certain on-order pixels in the IRS modules --- in particular, on the
antimony LL and LH arrays --- seem to vary on timescales of many hours
to days. Many of these are not marked in the default PMASK and BMASK
mask files which identify, respectively, permanent bad pixels, and
pixels which suffered from saturation, cosmic ray hits, etc. The
effectiveness of sky-subtraction at restoring on-scale many of these
rogue pixels is illustrated, and a proposed method for utilizing, in the
absence of dedicated sky observations, the routine IRS dark measurements
for such a purpose is outlined.

Dedicated Sky Subtraction

When close-in-time sky backgrounds observed in the same module and
exposure sequence are subtracted at the BCD level, not only is the sky
removed, but the majority of the hot or anomalous pixels are brought
back on scale. It is as if these pixels still show good response to
light, but have an induced offset which slowly varies. An example with
and without subtracting an average of 33 sky background LL BCDs:

LL order 1 (order at left):

Without:With:

LL order 2 (order at right):

Without:With:

All BCDs were taken over the course of ~1 hour. Some static bad
pixels do remain on order, but the majority are well recovered by this
type of subtraction. An ancillary benefit is the reduction/removal of
the LL1 fringing which results from filter delamination. Since the
background used in the subtraction was of such high signal-to-noise,
little additional noise was injected into the spectrum.

LH:

Without:With:

Since the data set we used to produce this did not have dedicated sky
frames, we used low flux frames as stand-ins, so some object
self-subtraction occurred. The crust visible on the edges of
the orders is an artifact of the LH flat, and is expected to be greatly
reduced with the flats slated for August, 2004. The remaining black
pixels are NaN'd in the original frames.

Suppression of rogue pixels is evident on-order, though not as
obviously as for LL. Suppression between orders is very good. Another
example using a low flux source, with 15 BCD frames used to create a sky
for subtraction from one of them:

LH:

Without:With:

A straight full-slit extraction before (colored) and after (white)
background subtraction illustrates the improvement:

Closest-in-time Dark Sky Subtraction

The subtraction of high quality, tailored background observations as
illustrated above will remain the best method to remove background
emission and treat these rogue pixels. However, many programs do not
have the luxury of dedicated concomitant background observations for
all modules and orders.

During routine IRS observations, dark sky measurements are taken at
regular intervals (originally, every 12 hours) to track pixel response
and other variations. Currently, these data are not used in the three
dimensional dark subtraction thread of the IRS pipeline; instead a
superdark constructed of many different dark cubes from older
observations of low background regions are used.

Depending on the typical time scale of rogue pixel variability, it
may be possible to use most-recent-in-time dark observations gathered
during routine IRS observations to perform a similar beneficial
subtraction as is found when dedicated backgrounds are used. Whether
this correction would be performed in toto on the entire BCD, or only on
selected problematic pixels remains to be seen.

We propose to test this hypothesis by collecting all darks for two
different IRS campaigns (one older, and one very recent) and applying
them to science data. We will explore the quality of the subtraction as
a function of time differential between dark and science observations,
and assess the typical S/N degradation imposed by the subtraction.

Currently, since dark subtraction occurs in 3D, darks are not run
through the science pipeline, and are thus not converted into the BCD
products which are suitable for this type of testing. For this
exercise, we need darks which have been treated as science data
(specifically as low-background zody+cirrus measurements). Presuming
they sample the same relative low background fluxes as do the
superdarks, the subtraction will not remove background from science
observations, but will hopefully contain useful information on the rogue
pixel pedestal values.